[QE-users] occupation in biological molecules and band gap

[Mahmoud Payami Shabestari]

Dear Xavier,



> So, I can use the Fixed occupation and, if the energy levels show a small 
bandgap, I introduce the >Smearing. Is it right?



Yes, exactly.



>If yes, I just have to add the nbnd function in the input file?



Yes. It suffices to put nbnd equal to N/2 + 1 (N is number of electrons).



Mahmoud

From: xavier neto via users <users at lists.quantum-espresso.org>
To: Mahmoud Payami Shabestari <mpayami at aeoi.org.ir>, Quantum ESPRESSO users 
Forum <users at lists.quantum-espresso.org>
Date: Mon, 1 Nov 2021 21:04:31 -0300
Subject: [QE-users] occupation in biological molecules and band gap



Dear Mahmoud Payami,


Thank you for your answer. It helps me a lot.


So, I can use the Fixed occupation and, if the energy levels show a small 
bandgap, I introduce the Smearing. Is it right?





About the second question. I've read that the result of HOMO - LUMO, 
obtained after the SCF calculation, can give the bandgap. Can I obtain the 
bandgap in that way? If yes, I just have to add the nbnd function in the 
input file?


On 11/1/21 02:04, Mahmoud Payami Shabestari wrote:
Dear José Xavier,

Hi.

Smearing for the occupations is used when the levels near HOMO are so close 
(commonly in metals near Fermi level) that in consecutive iterations they 
may exchange positions and lead to nonconvergence. For semiconductors with 
3-5 eV gap I think it won't help. 

Bests
Mahmoud Payami
NSTRI, AEOI, Tehran, Iran




From: José Xavier via users <users at lists.quantum-espresso.org>
To: "users at lists.quantum-espresso.org" <users at lists.quantum-espresso.org>
Date: Mon, 1 Nov 2021 00:18:55 +0000 (UTC)
Subject: [QE-users] occupation in biological molecules and band gap


Dear all,

I'm new in QE. The group that I'm working with usually calculates the 
properties of biologically relevant molecules, like amino acids, hormones, 
neurotransmitters, and drugs, and I would like to introduce the QE code to 
the group. Their previous papers have shown that the bandgap of these 
crystals is something between 3-5 eV, which I understand to be in the 
semiconductor range.

I've watched/read some tutorials about QE, and some of these showed that the 
occupation of semiconductors should be "smearing", other ones that it should 
be "fixed", but no one showed calculations with this kind of molecules. What 
value should I use for the SCF and electronic calculations of the crystals 
that I'm going to work on (amino acids, hormones, neurotransmitters, and 
drugs)?

*Are there any differences when the molecule has a metal, like the Heme 
group?

Besides, I would like to ask if there is a calculation that I could perform 
to obtain the bandgap of these molecules. Is it only possible if I create 
the band structure figure and calculate the difference between the peaks in 
VB and CB?

Thank you for your help,

Sincerely,
José Xavier

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